This invention relates to spatial modulation of infrared sources and, more particularly, to spatial modulators in which a portion of the beam forming optics is rotated and another portion is stationary.
Spatial modulators are used to collect radiation emitted from a source and generate rotating beams. Such modulators often comprise reflective optics. The reflective optics typically surround a source of radiation, collect the radiation from the source and form it into distinct beams. Modulation is achieved by rotating the beamforming optics.
Typically each section of reflective optics which creates a single beam has a cross section conforming to a portion of a parabola which has the source of radiation at its focus. The reflective optics are typically elongated in the vertical direction (along the longitudinal or major axis of an elongated radiation source) to provide a wide beam in elevation.
Spatial modulators of this type are employed in aircraft as a countermeasure against heat seeking missiles. In order to effectively countermeasure a heat seeking missile, it is necessary that the radiant intensity of the beam be high. To accomplish this, the reflective optics must collect the maximum amount of radiation emitted from the source. Because of the wide elevation angle of the beams, the average intensity at any one point in the beam is much less than that which could be obtained with beams of smaller elevation angles.
Another difficulty of designing such a system is that of preserving the projected area-solid angle product (etendue) of the source at the output of the collector. This insures the maximum radiant intensity in the far field which can be gotten within the angles of desired coverage from the given source.
Another one of the problems associated with this type of system is the size of the motors necessary to rotate the relatively large and heavy reflective optics.
Applications have now arisen where ships also require protection against heat seeking missiles. These missiles generally approach the ship just above the waterline and, thus, the countermeasuring beams do not need to have a wide elevation angle.